This invention relates to an agent for developing a latent electrostatic image formed on the surface of photoreceptors used in electrophotography, electrostatic recording, electrostatic printing and other reprographic methods.
Production of visible images from image information of interest is commonly accomplished by methods that involve the formation of latent electrostatic images, such as electrophotography, electrostatic recording and electrostatic printing. In electrophotography, a uniform static charge layer is first provided on the surface of a photoreceptor having a light-sensitive layer made of a photoconductive material. By subsequent imagewise exposure, a latent electrostatic image corresponding to the illuminated original is formed on the surface of the photoreceptor and then developed with a developer to form a toner image. The toner image is transferred onto a recording material such as paper and thereafter fixed by heating or pressure application to produce a copy image. After the transfer step, the photoreceptor is flooded with light to neutralize any residual charges and the toner image remaining on the photoreceptor is wiped off to condition the photoreceptor for another image cycle.
In order to insure that a copy image of high density without fog is formed over many cycles in a consistent way, the triboelectric charge on the toner particles must always be within an appropriate range. To this end, it is essential that fouling of the triboelectricity providing material or member (e.g. carrier or doctor blade) or the developer transport medium by the toner material be effectively prevented. Further, the fouling of the photoreceptor by the toner material must be prevented in order to insure an effective potential creation.
From the viewpoints of cleaning efficiency and durability, the toner that remains on the photoreceptor after the transfer step is preferably wiped off by means of cleaning with a doctor blade. However, the finer the particles of the toner that remains on the photoreceptor, the greater the force of relative adhesion to the photoreceptor and the more difficult it becomes to wipe off the toner. To prevent this problem of "insufficient cleaning", the toner is required to have good cleanability.
The toner image is preferably fixed by means of a hot pressure roll since it provides high heat efficiency and is capable of high-speed fixing. However, the use of a hot pressure roll as a fixing means has the disadvantage that "offsetting", a phenomenon in which part of the toner powder in the toner image being fixed is transferred onto the surface of the roll and is then transferred onto the recording material being sent in the next image cycle, whereby the final image is fouled. In order to prevent this trouble, the toner must have good anti-offsetting quality.
In continuous image formation, a substantial amount of heat is transferred to the recording material and the temperature of the hot pressure roll has a great tendency to drop, thus increasing the chance of the recording material of sticking to the surface of the roll. Thus, the toner is also required to have high resistance to this problem of "sticking".
With a view to satisfying the requirements described above, particularly anti-offsetting and anti-sticking qualities, it has already been proposed that a crosslinked polyester (Unexamined Published Japanese Patent Application No. 37353/1982) be used in combination with an incorporated low-molecular weight polypropylene (hereinafter abbreviated as PP; see Unexamined Published Japanese Patent Application No. 65231/1974) in toners. However, at high copying speeds or in small developing and cleaning devices, the combination of a polyester and a polypropylene has been found to be unsatisfactory since (1) the fluidity and agglomeration of toner particles is low and (2) the toner has a tendency to fuse to the triboelectricity providing material or member or the photoreceptor. This may be explained as follows: highly polar polyesters that have high contents of an ester group ##STR1## a carboxyl group (--COOH) and a hydroxy group (--OH) inherently have a very small force of interfacial adhesion to the nonpolar polypropylene, so that in the process of toner production, particularly in the pulverizing and classifying steps, fine particles of polypropylene (10-30 .mu.m) are formed to cause the problems (1) and (2) mentioned above. In addition, the dissociation of polypropylene is accelerated under the impact of compression by the cleaning blade or agitation in various pipes.
In an attempt to solve these problems, it has been proposed that polypropylene be oxidized or acid-modified to introduce carboxyl or hydroxy groups (see Unexamined Published Japanese Patent Application Nos. 54348/1983, 129863/1984, 226160/1987 and 229159/1987). However, this approach has had the following disadvantages:
a. A solid black image at the leading edge of the recording material has a great tendency to stick to the surface of a hot pressure roll;
1) combinations of acid-modified PP and common polyesters (PEs) as taught in Unexamined Published Japanese Patent Application Nos. 54348/1983, 229159/1987 and 226160/1987 have such a good miscibility between the components that phase separation does not readily occur in the fixing step, and this reduces the efficiency of paper release. This problem is particularly noticeable with a solid black image at the leading edge of paper and has been the major cause of troubles such as jamming in the fixing step;
2) the combination of an acid-modified PP and a polyester of bisphenol A type (Unexamined Published Japanese Patent Application No. 129863/1984) is a typical example of the case where the problem described in (1) is most likely to occur.
b. Fine toner particles will readily form in the developing device to impair developing, transfer and cleaning qualities;
1) methods are available that use acid-modified PPs that are highly miscible with polyesters in order to provide improved toner fluidity, agglomeration and resistance to fusion (inclusive of filming to the photoreceptor) (see Unexamined Published Japanese Patent Application Nos. 29159/1987 and 226160/1987), but polyester resins themselves are brittle and have a tendency to disintegrate to form fine toner particles in the developing device as the copying cycle is repeated; as a result, the relative adhesion between the triboelectricity providing material or member and the toner and that between the photoreceptor and the toner will increase to cause deterioration in developing, transfer and cleaning qualities; further, the fine toner particles will fuse to the triboelectricity providing material or member in copying cycles, whereby the charging ability of said material or member is reduced; and
2) the already described combination of an acid-modified PP and a polyester of bisphenol A type (Unexamined Published Japanese Patent Application No. 129863/1984) is again a typical example of the case where the problem just described above in (1) is most likely to occur.